The present invention relates to the manufacture of ceramic matrix composite products, and more particularly to an improved method for consolidating such composites to provide products of high density and strength.
Ceramic matrix composites may be characterized as composites wherein a ceramic matrix material such as a glass, glass-ceramic or crystalline ceramic is reinforced by the inclusion of inorganic fibers or whiskers. The reinforcement may consist of long fibers, short or chopped fibers, or whiskers, these being typically composed of silicon carbide, mullite, alumina or other material available in fiber or whisker form.
Ceramic composite materials are of interest because they exhibit higher strength and toughness than ordinary ceramic materials. Thus they are less prone to sudden catastrophic brittle failure than conventional ceramics.
Ceramic matrix composites are customarily fabricated by combining the selected reinforcing fiber material with finely divided fibrous or powdered glass or other ceramic material. The combination of fibers and ceramics, normally in the form of a unitary porous preform, is then consolidated with heat and pressure. In order to realize the potential high strength and toughness of these materials, consolidation must be complete, i.e., very few voids or defects should remain in the consolidated material. This requires the application of substantial heat and pressure to the powdered ceramic, utilizing temperatures well above those required for the processing of organic or metallic materials.
Hot pressing has been the process of choice for the fabrication of fiber-reinforced ceramic matrix composites because it is capable of applying unidirectional pressures greater than 1000 psi at temperatures above 1000.degree. C. This is the forming regime in which most composites comprising glass or glass-ceramic matrix materials must be consolidated. U.S. Pat. No. 4,764,195, for example, describes some of the processing parameters to be observed in the processing of glass-matrix composites. However, while hot pressing has served well for the initial feasibility evaluations of these materials, there is a clear need to develop forming methods adaptable to the formation of more complex shapes in fiber-reinforced ceramic composites.
It is well known that relatively small ceramic parts can be formed from cold-pressed powder preforms of near net shape by hot isostatic pressing. U.S. Pat. Nos. 4,339,271 and 4,112,143, for example, describe processes wherein such parts are simply coated with powders of one or more fusible glass or metallic materials, and then finally consolidated, after fusion of the fusible powder coating into a gas barrier, by the application of heat and pressure to the coated compacts. Unfortunately this method cannot be used with larger unpressed parts such as ceramic matrix composite preforms. The high incidence of pinholes in the powder coatings, which effectively prevent consolidation of the part, makes large part fabrication by this method totally impracticable. In addition, the risk of part distortion, due to the larger preform sizes and relatively high degree of preform compaction required, is considerable.
Ceramic matrix composites can be classified into two different categories, depending upon the viscosity at which consolidation is most preferably carried out. In the low viscosity regime are most glass matrix composites. These are typically consolidated at temperatures near the working point viscosity of the matrix glass, e.g., 10.sup.3 -10.sup.4 poises.
In the high viscosity consolidation regime are composites comprising glass-ceramic matrix materials. Due to the thermal crystallization behavior of these materials, they are more typically consolidated at viscosities of 10.sup.7 -10.sup.12 poises, viscosities which are near the softening point of the glasses prior to crystallization. It should be noted that, although the processing concepts in these two regimes are similar, the consolidation mechanics are significantly different.
A critical consideration to be addressed in the processing of these composites is the very high degree of debulking which must be achieved. Typical preforms for ceramic matrix composites are about 25% dense, i.e., they occupy four times the volume which the fully densified ceramic composite will occupy after consolidation is completed. Accordingly, for any complex shape, there is a likelihood that a great deal of fiber realignment, inter-ply slippage or the like will occur during consolidation as the fibers are compelled to conform to the surfaces of the consolidation molds. For this reason the consolidation of complex shapes requires the careful design of the consolidation process and equipment, as well as the design of the preform, in order that the reinforcing fibers not be placed in compression to cause fiber buckling or ply wrinkling.
It is a principal object of the present invention to provide an improved consolidation process for the manufacture of ceramic matrix composites which addresses some of the aforementioned difficulties.
It is a further object of the invention to provide a consolidation process which will permit the consolidation of composites of complex shape at high temperatures and pressures.
Other objects and advantages of the invention will become apparent from the following description thereof.